3 Answers
3

First, I am not a lawyer, this is not legal advice, if you follow my advice and get sued and lose your house then it's your fault not mine.

Unfortunately, as simple as that question is, it really needs to be broken down into two questions.

1) If I get sued for copyright infringement, will I win?

Maybe! Assuming those pieces are in fact public domain now, then it seems like a rather clear victory. Now, for the sake of this entry, I'm going to assume you win, unquestionably. I don't know that for certain, but we're just going to make that assumption.

2) Will I get sued?

Maybe.

Unless you have a nice big legal war chest to rely on, getting sued is almost as bad as losing. You can't hope to match Lego's legal funds. It will not happen. You'll go to court, and you'll probably blow thousands of dollars just on prep work. And then, even if you win, there's no way you're getting that money back without a second court case - more money, and you're not even guaranteed to win that one.

You may want to read this story. For posterity, here's the important bits:

As a film student in early 2001, I was a juggernaut. I was making a
lot of short films that were garnering moderate acclaim, and I was
always pushing ahead, making bigger and bigger projects. For my senior
thesis, I wanted to base it on a short story by Isaac Asimov, which
was part of the compilation that made up the book “I, Robot”.

I called, I got permission, and they faxed the form to my professor’s
office.

In our last week of shooting, 3 months after I received written
consent to use the short story, one of the crew brought in a copy of
Variety, which mentioned that a major studio purchased the book rights
to I, Robot, and planned to make a film. Initially, I thought,
“Awesome – free promotion!” Alas, that’s not what was looming on the
horizon.

Somehow the legal team from the studio found out about a student
project, in a small private college in the Midwest, with no budget,
being shot in a warehouse basement, and decided to issue a cease and
desist order. Basically, what that means, is that the studio’s lawyers
said to us, “You’re using our property. Stop, or we’ll sue you into
the stone age.” I responded by sending them the consent form from the
Asimov estate, and explained that it was a student project, not a
commercial venture worth litigating.

Instead of the letter recognizing our valiant efforts as students that
I expected, I found myself on the tail end of a phone call that
changed my life. I was contacted directly by the lead of the studio’s
legal team, who explained my situation to me very clearly. He told me
that I was technically in my legal right to use Isaac Asimov’s
material. However, if I chose to proceed, they would file multiple
lawsuits totaling over 2 million dollars against me. In the end, I
might win, but it would take hundreds of thousands of dollars in legal
fees just to fight it, but would cost them nothing more than the
salaries they already pay their lawyers. It would be 10 years before
any type of verdict could be levied, and by then it wouldn’t matter
what the outcome was, since their film would be long since released.

So, when you say "Well, I can, but is it legal?", that's really the wrong question to ask. The right question is whether or not Lego will permit you to do so, and whether you have the money to defend yourself if Lego decides they won't let you do so.

This kind of questions are usually answered by lawyers after a good amount of money, and not even them will tell you for sure. To be sure you'll have to meet a judge in a courtroom, which I guess is what you're trying to avoid :)

If Lego is actually public domain you'll probably have no problems as long as you don't use the Lego trademark/logo or anything else that isn't public domain.

Trademark is easy enough to avoid (like you said, just don't use the Lego name/logo anywhere), but patents may be more complicated. Even though the Lego brick patent has expired ages ago, there may be more recent software patents associated with computer simulations/games of the Lego bricks.
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SkimFluxFeb 17 '12 at 14:37

@SkimFlux that's what I meant. There are always corner-cases which only a judge in a courtroom can decide. Can you patent "a game with lego blocks"? Probably not in Europe, but I'm unaware of the US legislation.
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kaoDFeb 17 '12 at 14:55

Just because the basic LEGO patent has expired, that doesn't mean that all their other intellectual property is in the public domain, nor that the laws are the same everywhere you want to market your own product. If your game represents itself as being in any way remotely related to LEGO toys, the LEGO company or trademarks, you could face trouble.

A game that has blocks that have the LEGO logo on them will definitely be a problem.

A game that uses bricks that look a lot like LEGO could be a problem.

A game that uses interlocking bricks with a vague resemblance to LEGO will probably be fine.

A game that uses interlocking bricks that don't look like LEGO bricks will almost certainly be fine.

A game with a generic construction theme would be fine.

If in doubt, come up with your own slant on the construction idea, or pay for proper legal advice.